1. Field of the Invention
The present invention relates to an electromagnetic interference shield and ground cage, used within fiber optic data communications transceivers for example, and, in particular, to an electromagnetic interference shield that significantly improves the electromagnetic interference (EMI) susceptibility of an optical receiver, for example, and which also serves as a low impedance ground cage for the optical transceiver.
2. Background Information
An optical transceiver is a device that uses pulses of light to carry signals and transmit and receive data at very high speeds. Typically, the light pulses are converted into, or converted from, associated electrical signals using known circuitry. Such optical transceivers are often used in devices, such as computers and data communication networks, in which data must be transmitted at high rates of speed.
Optical transceivers typically include an optical transmitter, such as a light emitting diode (LED) or laser, for example, to transmit the light pulses, and/or an optical receiver, such as a photodiode or photo detector, for example, to receive the light pulses. The optical receiver may be located adjacent to the optical transmitter to form a so-called duplex optical transceiver, such as when a so-called electro-optic receiver optical subassembly (hereinafter ROSA for short) is contained within a package, and positioned next to or adjacent to the transmitter optical subassembly (hereinafter TOSA for short). Alternatively, the optical receiver may be disposed separate from the optical transmitter.
Fiber optic transceivers typically are designed and deployed in the duplex optical transceiver configuration, comprised of both the transmitter and receiver optical devices (laser and photo detector, for example) and their associated electronics (laser driver and control, receiver preamplifier and post amplifier, and other supporting components). This allows two transceivers, separated over a distance and connected through a duplex fiber cable, to talk to one another.
Since about 1990, the fiber optic industry has been using a so-called SC duplex fiber optic connector system as the optical fiber connector interface on front of fiber optic transceivers (GBICs, SOCS, GLMs, 1X9s, etc.). The physical separation between the transmitter and receiver optical subassemblies (TOSA and ROSA) for the SC duplex connector is about 12.7 mm. However, the industry is now converting to the so-called Small Form Factor (SFF) optical connectors and associated SFF optical transceivers. For a so-called SFF LC optical connector, the separation between the TOSA and ROSA is about 6.25 mm, less than half that of the SC duplex connector. The reduced separation between the receiver and the adjacent transmitter, within a transceiver package, increases the strength of the electromagnetic coupling (or cross-talk) from the transmitter to the sensitive receiver. In terms of strength of the high speed (for instance 1 Gb/s) signal transitions, the laser driver delivers one volt signal transitions into the laser, while the adjacent sensitive receiver is delivering about 20 mV signal transitions to the post amplifier. Thus, a means of isolating or shielding the receiver from the transmitter electromagnetic radiation is needed. Strengthening or hardening the receiver against the transmitter radiation also improves its susceptibility to other EMI sources, such as emissions which radiate from within the computer system in which the transceiver modules are mounted, or from an adjacent module.
In either case, fiber-optic cables are coupled to the respective optical transmitter, and to the optical receiver, so that the light pulses can be transmitted to and from other optical transceivers, for example.
The optical transceivers are normally located on either input/output printed circuit cards, or on port cards that are connected to an input/output card (hereinafter, the card to which the optical transceiver is connected will be referred to as the host printed circuit board, or host PCB). In order to facilitate the connection of the fiber-optic cable to the optical transceiver, the transceiver is usually located on a periphery of the host printed circuit board.
Moreover, in a computer system, for example, the host printed circuit board (with the optical transceiver attached thereto) is typically connected to a further circuit board, for example, a motherboard. The assembly may then be positioned within a chassis, which is a frame fixed within a computer housing. The chassis serves to hold the assembly within the computer housing.
Typically, the optical transceiver contains its own printed circuit board (hereinafter transceiver PCB) on which the transceiver electronics (laser driver, post amplifier, etc.) are mounted, forming the interface or connection between the TOSA and ROSA (connected to the transceiver PCB) and the host PCB. The TOSA and ROSA are connected to the transceiver PCB using a number of leads, for example, when lasers or receivers are mounted in TO-cans, having a circular geometry of about 5 mm diameter. For example, the aforementioned electro-optic receiver optical subassembly (ROSA) conventionally has four leads: a power lead for supplying power to the ROSA; a single ground lead for connecting the ROSA to a ground potential; and two data leads for transmitting signals to and/or from the ROSA. Each of the four leads is typically directly connected to the transceiver printed circuit board in a known manner. For example, the ends of the respective leads may be passed into corresponding vias formed in the printed circuit board, and soldered in place. Alternatively, the four ROSA leads may be edge mounted or connected to the transceiver PCB by soldering to electrical pads on the bottom or top of the transceiver PCB. Further, each of the four leads typically has a relatively long length. The long lengths have generally been deemed necessary in order to allow the ROSA to be properly oriented relative to the transceiver PCB, while still allowing the ROSA to be properly connected thereto. This is because the leads typically extend out of a rear portion of the ROSA and initially in a direction parallel to the surface of the transceiver printed circuit board. Thus, in order to connect the leads to the transceiver printed circuit board, the leads must extend for a distance in a different direction and toward the printed circuit board. Depending on the orientation and geometry, the relatively long length of the single ground lead, for example, causes the ground lead to disadvantageously have a relatively high impedance. Also, since the ground lead is attached and connected to the TO-can body, it is more exposed to being hit by impinging EMI radiation. As is known to those skilled in the art, a high impedance on the receiver power or ground leads is undesirable, since this affects the immunity of the receiver to noise present on the power supply or ground. Therefore, there is a need to provide a way of grounding an optical receiver, for example, at a low impedance.
Furthermore, many electrical devices, when operated, generate emissions that include electromagnetic radiation. When this electromagnetic radiation influences the proper functioning of another device, the result is known as electromagnetic interference (also known as EMI).
Various shield devices are known that can be used to reduce emitted electromagnetic radiation or protect or harden a device against emissions that impinge on it from another source (radiated electromagnetic susceptibility (RES), for example, from the adjacent transmitter or other sources of EMI radiation. The conventional shields typically cover a substantial portion of the associated electrical device, and are usually formed of a to metal that, when grounded, will attenuate or redirect the interfering electromagnetic radiation.
To prevent electromagnetic interference from having an adverse effect on the sensitive optical receiver, it is known to provide a card-mounted shield that covers the leads, for example, of the optical receiver in order to reduce the amount of electromagnetic radiation that is coupled onto the receiver leads. In a similar fashion, a shield can be attached to the transmitter optical subassembly (TOSA), surrounding its leads to reduce the amount of electromagnetic radiation that is emitted from the transmitter. These shields are typically attached and grounded to the transceiver PCB, which in turn is fastened to, and grounded in a known manner, to the host PCB and its ground.
However, the conventional optical transceiver shield, when properly positioned over a standard optical transceiver, does not prevent cross-talk (i.e., undesired coupling) between the transmitter data leads and the sensitive data leads or ground lead of the optical receiver. This is because the data leads and the ground lead are all located essentially parallel and adjacent to each other, and are all disposed inside the shield, which is also grounded. The transmitter emissions interfere with the receiver shield and ground, and then interfere with the receiver data leads by passing through the power supply of the receiver. Thus, the shield does not separate (nor shield) the data leads from the ground lead. Thus, there is a need to provide a shield that will prevent cross-talk from the transmitter to the receiver ground.
Moreover, the conventional optical receiver or transmitter shield, if not properly positioned, may inadvertently contact either the data leads or the power lead, thus causing a short circuit. Thus, there is a need for a shield that can be properly aligned relative to the leads of the transceiver, to prevent the leads from shorting out.
It is, therefore, a principle object of this invention to provide an electromagnetic interference shield and ground cage.
It is another object of the invention to provide an electromagnetic interference shield and ground cage that solves the above mentioned problems.
These and other objects of the present invention are accomplished by the electromagnetic interference shield and ground cage disclosed herein.
According to one aspect of the invention, each of the side, back and front walls of the electromagnetic interference shield and ground cage are integral with the top wall. This advantageously allows all the walls of the electromagnetic interference shield and ground cage to be simultaneously stamped or cut from a sheet of steel, for example, and then bent into the desired configuration.
In a further exemplary aspect of the invention, the abutting edges of the back wall and side walls are provided with intermeshing teeth. When the walls are properly positioned relative to each other, the intermeshing teeth of the respective walls will engage, thus advantageously reducing any gaps that might otherwise be formed between the abutting edges. As will be appreciated, gaps opening directly into the electromagnetic interference shield and ground cage may disadvantageously allow for the passage of electromagnetic interference.
In another aspect of the invention, the electromagnetic interference shield and ground cage has at least two conductive ground connection pins disposed on a lower edge of the walls. The connection pins can be easily inserted into vias formed in a printed circuit board, for connection with a ground layer by soldering, for example. Further, the connection pins may be integral with the walls to which they are connected. This advantageously allows all the connection pins to be stamped or cut from a sheet of steel, for example, simultaneous with the forming of the walls.
In another exemplary aspect of the invention, the ground lead of an electrical component projects into an opening formed in a front wall of the electromagnetic interference shield and ground cage. The ground lead does not extend past the opening any substantial distance. Instead, the ground lead is electrically coupled to the electromagnetic interference shield and ground cage by soldering, for example, the ground lead to the front wall at the opening. Any remaining portion of the ground lead that extends past the opening may then be removed. As will be appreciated, this will prevent the ground lead from being directly connected to a printed circuit board in the conventional manner. However, since the electromagnetic interference shield and ground cage is connected to a ground potential of a printed circuit board, and since the length of the ground lead is reduced to essentially zero, the impedance through the ground connection is advantageously reduced. Moreover, since the electromagnetic interference shield and ground cage has a substantially larger surface area than the original ground lead, an improved high frequency ground connection is provided, and the inductance of the ground connection is reduced, thus likewise reducing cross-talk with the other leads and other components.
In another aspect of the present invention, an insulator clip, formed from plastic, for example, is attached to the front wall. The insulator clip is provided with a relatively flat base portion, which fits flush on an outer surface of the front wall. The base portion has an opening, which is positioned over the opening in front wall. The insulator clip is further provided with two resilient protruding clips disposed on opposite sides of the opening in the base portion. The protruding clips project through the opening in the front wall, and catch on an inner surface of the front wall to hold the insulator member in position. When the power lead is inserted through the opening in the base portion, the protruding clips advantageously prevent the power lead from inadvertently coming into contact with the front wall, or in contact with the data leads. Thus, the insulator clip advantageously prevents the power lead from accidentally shorting out.
Moreover, preferably the holes in the front wall for the data leads are sized larger than the opening through the base portion. Thus, the insulator clip helps to properly align the electromagnetic interference shield and ground cage relative to the electrical component, thus minimizing the possibility that the data leads will inadvertently contact the front wall and short to ground.
Further, the base portion advantageously serves as a spacer between the front wall and the electrical component, which prevents adhesives used during the assembly of the ROSA or TOSA, for example, from mechanically interfering with attachment of the shield.